In orthogonal frequency division multiple access (OFDMA) communication systems, the total bandwidth is divided into non-overlapping frequency blocks also called resource blocks (RBs), where transmissions for user equipment (UE) occur in an orthogonal and not mutually interfering manner. Each RB carries data for a specific UE. By scheduling each UE on RBs where it has a high signal to interference and noise ratio (SINR), the data rate may be maximized according to a specific scheduling criterion. To enable frequency-domain scheduling and allocate UEs on RBs with good SINR, each UE feeds back a channel quality indicator (CQI) to its serving base station (Node B).
For MIMO communication with downlink precoding, a pre-defined codebook is designed offline and known at the Node B and UE. The codebook consists of a set of matrices UεCN×N, where N is the number of transmit antennas and R is the transmission rank that determines the number of data layers multiplexed in the spatial domain. Both the transmission rank R and the preceding matrix U may be chosen to optimize performance on each RB. However, in many practical systems such as 3GPP LTE, the transmission rank R is fixed on all RBs for a given UE. The preceding matrix, however, could vary from one RB to the other, even for the same UE.
In order to find an optimum rank and preceding matrix, a UE applies the estimated CQI to exhaustively search all ranks and all preceding matrices in each rank codebook. A preferred transmission rank R and preceding matrix U are chosen to optimize a certain optimality metric such as the transmission throughput according to the specific scheduling criterion. The preferred rank and precoding matrix index are then fed back or reported to the Node B. Improvements in the selection and feed back of rank and preceding requirements would prove beneficial in the art.